Ring communication systems which use a token mechanism for access to the transmission ring and which allow asynchronous as well as synchronous transmission of information are well known already, e.g. from the following patents and publications:
U.S. Pat. No. 4,429,405 entitled "Method of transmitting information between stations attached to a unidirectional transmission ring". - U.S. Pat. No. 4,482,999 entitled "Method of transmitting information between stations attached to a unidirectional transmission ring". - U.S. Pat. No. 4,539,679 entitled "Synchronization in a communication network of interconnected rings". - W. Bux et al.: "A local-area network based on a reliable token-ring system"; published in "Local Computer Networks", North-Holland Publ. Co. 1982, pp. 69-82. - N. C. Strole: "A local communication network based on interconnected token-access rings: A tutorial", IBM Journal of Research and Development, Vol. 27, No. 5, September 1983, pp. 481-496.
In the systems disclosed in above cited patents and publications, priority tokens are issued at regular intervals to allow the transmission of such information which needs a synchronous transmission service. If the synchronous information, e.g. digital voice samples, is to be transferred between different rings which are interconnected by bridges, or between a ring and another network through a gateway unit, additional delays may be encountered in these bridges or gateways which is undesirable for this type of information.
For the transmission of synchronous information, other ring systems are known which are of the slotted or time division multiplex type in that they use a fixed time raster of slots and frames. Examples of such systems are given in the following patents: U.S. Pat. No. 4,154,983 entitled "Loop carrier system for telecommunications and data services". - Swiss Pat. No. 502,043 entitled "Communication system, particularly telephone system, for time division multiplex operation". These systems, though well-suited for the transmission of voice between attached stations, are less effective for the transmission of data blocks or files of irregular length because of the fixed time raster or TDM scheme.
Exchange systems of the PBX type for time division multiplex switching of voice or data between a number of ports were disclosed e.g. in the following patent and publications: U.S. Pat. No. 3,856,993 entitled "Time division multiplex exchange". - J. M. Kasson: "The Rolm computerized branch exchange: An advanced digital PBX", Computer, June 1979, pp. 24-31. - J. M. Kasson et al.: "The CBX II switching architecture", IEEE Journal on Selected Areas in Communications, Vol. SAC-3, No. 4, July 1985, pp. 555-560. None of the switching systems disclosed provides for the immediate interconnection to a ring network with basically asynchronous operation or with a token access mechanism.